printers: Tweak the printer supply level bar

Make the supply level bar size, margin, border, background, and
saturation match the mockups at
https://wiki.gnome.org/Design/SystemSettings/Printers#Guidelines

Even though we would obtain primary colors by querying the printer
supply info, it is more eye-candy to desaturate (tone down) the
colors in the UI.

https://bugzilla.gnome.org/show_bug.cgi?id=779075